The present invention relates to communication over a network. More specifically, the present invention relates to a method and apparatus for identifying a digital data channel from among a plurality of channels used in a data-over-cable system.
With the explosive increase in the use of the Internet and other computer networks, many customers have desired network connections that provide a higher data rate than can be supported by a modem used over a standard telephone line. One approach has been to use the larger bandwidth of a cable television network to transmit data between customers"" computers and the Internet and/or other computer networks. In such cable television networks, the signals are carried by cables, which are typically coaxial cables and/or fiber optic cables. Many cable television networks are Hybrid Fiber/Coaxial (xe2x80x9cHFCxe2x80x9d) cable systems, in which fiber optic cables are used for the backbone, and coaxial cables are used to connect the subscribers to the backbone.
Cable television networks typically use a passband ranging from about 54 MHz to over 300 MHz for the transmission of signals in the downstream direction, i.e., to the customers. The signals typically include analog television signals, which may be in the NTSC, PAL, or other format. The signals may also include digital television signals, such as a high definition television (xe2x80x9cHDTVxe2x80x9d) format. The passband is normally divided into a series of frequency channels, in accordance with a predetermined xe2x80x9cchannel planxe2x80x9d or xe2x80x9cfrequency plan.xe2x80x9d Cable television networks in the United States typically use one of three channel plans, the xe2x80x9cstandardxe2x80x9d channel plan, the Incrementally Related Carrier (xe2x80x9cIRCxe2x80x9d) channel plan, or the Harmonically Related Carrier (xe2x80x9cHRCxe2x80x9d) channel plan. The xe2x80x9cstandard plan,xe2x80x9d which is set forth in the Electronic Industry Association""s Interim Standard IS-6, provides a series of 6 MHz channels, with the lower edge of each channel set at an integral number of megahertz. For example, the lower edge of xe2x80x9cChannel 2xe2x80x9d in the xe2x80x9cstandardxe2x80x9d plan is 54 MHz. The channels in an IRC channel plan are also 6 MHz, and their frequencies are largely the same as those in the xe2x80x9cstandardxe2x80x9d plan. However, the IRC channel plan provides channels in the 72 MHz to 90 MHz range, whereas the xe2x80x9cstandardxe2x80x9d plan does not. In an HRC channel plan, the channels are also 6 MHz wide, but it is the visual carrier that is set at an integral number of megahertz in each channel, rather than the lower channel edge. Thus, the lower edge of xe2x80x9cChannel 2xe2x80x9d is 52.75 MHz, with the visual carrier at 54 MHz.
When such cable television networks are used to connect customers to computer networks, such as the Internet, one or more of the channels in the channel plan are used as xe2x80x9cdata channelsxe2x80x9d to transmit data from the computer network to the customers. Typically, other channels are still used to transmit television signals in the downstream direction. When cable television networks are used to transmit data in this way, they become part of what is often referred to as a xe2x80x9cData-Over-Cable System.xe2x80x9d In such systems, cable modems, such as those provided by 3Com Corporation of Santa Clara, Calif., and others, provide the interface between the cable television network and the customers"" computers. A cable modem is able to receive data transmitted downstream over the cable television network and forward it to a customer""s computer. A cable modem is, typically, also able to send data from a customer""s computer in an upstream direction, either via the cable television network or the public switched telephone network (xe2x80x9cPSTNxe2x80x9d). When used with cable television networks, cable modems are typically able to support a data connection to the Internet and other computer networks with a data rate of up to 30+Mbps, which is a much larger data rate than can be supported by a modem used over a standard telephone line.
Data-over-cable systems typically operate in accordance with the interim specifications set forth in the Data-Over-Cable Service Interface Specificationsxe2x80x94Radio Frequency Interface Specification (xe2x80x9cDOCSISxe2x80x9d), issued by Cable Television Laboratories, Inc. A recent version of DOCSIS, document control number SP-PFI-104-980724, dated Jul. 24, 1998, is incorporated herein by reference. DOCSIS may be found today on the World Wide Web at the Universal Resource Locator (xe2x80x9cURLxe2x80x9d) xe2x80x9cwww.cablemodem.comxe2x80x9d.
When a cable modem is connected to a data-over-cable system, it must undergo an initialization procedure before it is able to transmit and receive data. As part of the initialization procedure, the cable modem attempts to find a data channel with which it can synchronize. A difficulty with this part of the initialization procedure is that when a cable modem is first used, it typically does not know which of the many possible channels will be a data channel it can use, or even what channel plan will be applicable. In fact, most of the possible channels in typical data-over-cable systems are used for television signals or are not used at all.
To find a useable data channel, most cable modems scan through the possible channels for each of the three-commonly-used channel plans, xe2x80x9cstandard,xe2x80x9d IRC, and HRC. In this approach, the cable modem tunes, typically in a predetermined pattern, to each channel specified by a given channel plan, attempts to demodulate the channel, and then access the transported data. This approach can be time-consuming, because the modem must spend time attempting to demodulate channels that may not be data channels and because all three channel plans may need to be scanned in order to obtain the data channel. Additionally, this approach may not work in countries that use a channel plan that is not one of the three described above or when the data-over-cable system uses a non-standard channel plan.
In a first principal aspect, the present invention provides a method for selectively receiving a target signal component from among a plurality of signal components that make up a signal transmitted over a cable network to a receiver in a cable telecommunications system. Each of the signal components has a distinct frequency range. In accordance with the method, a plurality of power measurements are obtained for a first signal component, and the first signal component is identified as being the target signal component if the plurality of power measurements satisfy predetermined criteria. The plurality of power measurements includes a first power measurement and a second measurement. The first power measurement is obtained by: (a) tuning a frequency selection system to a first center frequency within the frequency range of the first signal component, so that the frequency selection system provides a first filtered signal from a first band of frequencies centered about the first center frequency; and (b) measuring, the power of the first filtered signal to obtain the first power measurement. Similarly, the second power measurement is obtained by: (a) tuning the frequency selection system to a second center frequency within the frequency range of the first signal component, so that the frequency selection system provides a second filtered signal from a second band of frequencies centered about the second center frequency; and (b) measuring the power of the second filtered signal to obtain the second power measurement.
In a second principal aspect, the present invention provide a method for selectively receiving a digital signal component from a plurality of signal components that make up a signal transmitted over a cable network to a cable modem in a data-over-cable system. The signal components are defined by a plurality of predetermined frequency channels, each having a channel bandwidth. The signal components include at least one analog signal component and at least one digital signal component, with the analog signal component being defined by analog channels and the digital signal components being defined by digital channels. The method includes the step of classifying a first channel based on the spectral component of any signal component present therein. To classify the first channel, a frequency selection system is tuned to a first center frequency within the first channel. The frequency selection system has a bandwidth substantially less than the channel bandwidth. The frequency selection system provides a first filtered signal from a first band of frequencies centered about the first center frequency, and the power of the first filtered signal is measured to obtain a first power measurement. Then, the frequency selection system is tuned to a second center frequency within the first channel. The frequency selection system provides a second filtered signal from a second band of frequencies centered about the second center frequency, and the power of the second filtered signal is measured to obtain a second power measurement.
In a third principal aspect, the present invention provides a cable modem for selectively receiving a digital signal component from a plurality of signal components that are transmitted as a signal over a cable network. The cable modem comprises a tunable frequency selection system, a demodulator, and a power measurement system. The frequency-selection system provides a filtered signal from a selected band of frequencies of the signal, and the frequency selection system is operable at a first bandwidth and a second, narrower, bandwidth. The demodulator obtains a digital signal by demodulating the filtered signal from the frequency selection system. The power measurement system provides a power level signal indicative of a power level associated with the filtered signal from the frequency selection system.
In a fourth principal aspect, the present invention provides a method for initializing a cable modem that is disposed to receive a signal transmitted over a cable network. The initialization process includes the following steps. The cable modem selectively receives a first band of frequencies centered about a first center frequency. The first band of frequencies has a first bandwidth. The cable modem receives a second band of frequencies centered about a second center frequency. This second band of frequencies has a bandwidth substantially equal to the first bandwidth. The cable modem then selectively receives a third band of frequencies centered about a third center frequency. The third band of frequencies has a bandwidth substantially greater than the first bandwidth. The third band of frequencies includes frequencies from the first band of frequencies and the second band of frequencies. The cable modem demodulates this third band of frequencies to obtain a digital signal, and the cable modem synchronizes with the digital signal.
In a fifth principal aspect, the present invention provides a method for selectively receiving a target signal component from among a plurality of signal components that make up a signal transmitted over a cable network to a cable modem in a data-over-cable system. The signal components are defined by a plurality of predetermined frequency channels. The cable modem has a tunable frequency selection system operable at a wide bandwidth and at a narrow bandwidth. The method includes the following steps. While operating at the narrow bandwidth, the frequency selection system tunes to a first frequency in a given one of the frequency channels and provides a first filtered signal from a first band of frequencies centered about the first frequency. The power of the first filtered signal is measured to obtain a first power measurement. While operating at the narrow bandwidth, the frequency selection system tunes to a second frequency in the given frequency channel and provides a second filtered signal from a second band of frequencies centered about the second frequency. The power of the second filtered signal is measured to obtain a second power measurement. If the first and second power measurements both exceed a predetermined level, the frequency selection system then operates at the wide bandwidth to provide a third filtered signal from a third band of frequencies in the given frequency channel.
In a sixth principal aspect, the present invention provides a method for identifying a possible plan in a cable telecommunications system in which a transmitter transmits a signal over a cable network to a receiver and the signal comprises a plurality of signal components defined by a plurality of predetermined frequency channels. The frequency channels have a channel bandwidth. The method includes the following steps. A frequency selection system tunes to a first frequency to provide a first filtered signal from a first band of frequencies of the signal centered about said first frequency. Tile first frequency corresponds to an expected frequency for a visual carrier in a first channel plan. The power of the first filtered signal is measured to obtain a first power measurement. The frequency selection system tunes to a second frequency to provide a second filtered signal from a second band of frequencies of the signal centered about said second frequency. The second frequency corresponds to an expected frequency for a visual carrier in a second channel plan. The power of the second filtered signal is measured to obtain a second power measurement. If the first power measurement exceeds the second power measurement by at least a predetermined amount, then the first channel plan is identified as the possible channel plan. If the second power measurement exceeds the first power measurement by at least a predetermined amount, then the second channel plan is identified as the possible channel plan.